Lower Critical Solution Temperature Behavior in Polyelectrolyte Complex Coacervates

Abstract

In light of recent experimental observations of lower critical solution temperature (LCST) in polyelectrolyte complex coacervates (Ali, S. et al. <i>ACS Macro Lett</i>. 2019, <i>8</i>, 289-293), we explore its possible mechanisms on the basis of a slight modification of our theory (Adhikari, S. et al. <i>J. Chem. Phys</i>. 2018, <i>149</i>, 163308). We explore the consequences of the temperature dependence of the solvent dielectric constant (<i>ε</i>) and the solvent-polymer interaction parameter (<i>χ</i>) on the complex coacervates' phase behavior. The results show that the temperature dependence of the solvent dielectric constant and solvent-polymer interaction parameter can result in a complex phase behavior involving two disjoint unstable regions on the temperature (<i>T</i>)-polyelectrolyte concentration (<i>ϕ</i> _p) plane. Comparison of phase diagrams constructed for different possible temperature dependencies of <i>ε</i> and <i>χ</i> shows that the experimentally observed LCST behavior is obtained only if the solvent dielectric constant decreases and the solvent-polymer interaction parameter increases with increasing temperature. Preferential partitioning of salt into the polyelectrolyte poor phase is predicted for all possible combinations of temperature dependencies of <i>χ</i> and <i>ε</i> considered in this work.